The present invention relates to lubricants used with refrigerants. More particularly, the present invention relates to lubricants for use with tetrafluoroethane, and preferably, 1,1,1,2-tetrafluoroethane (known in the art as R134a). R134a is a refrigerant which may replace dichlorodifluoromethane (known in the art as R12) in many applications because environmental concerns over the use of R12 exist.
R134a has been mentioned as a possible replacement for R12 because concern over potential depletion of the ozone layer exists. R12 is used in closed loop refrigeration systems; many of these systems are automotive air-conditioning systems. R134a has properties similar to those of R12 so that it is possible to substitute R134a for R12 with minimal changes in equipment being required. The symmetrical isomer of R134a is 1,1,2,2-tetrafluoroethane (known in the art as R134); the isomer is similar in properties also and may also be used. Consequently, it should be understood that in the following discussion, "tetrafluoroethane" will refer to both R134 and R134a.
A unique problem arises in such a substitution. Refrigeration systems which use R-12 generally use mineral oils to lubricate the compressor; the present discussion does not apply to absorption refrigeration equipment. See for example the discussion in Chapter 32 of the 1980 ASHRAE Systems Handbook. R-12 is completely miscible with such oils throughout the entire range of refrigeration system temperatures, which may range from about -45.5.degree. C. to 65.6.degree. C. Consequently, oil which dissolves in the refrigerant travels around the refrigeration loop and generally returns with the refrigerant to the compressor. The oil does not separate during condensation, although it may accumulate because low temperatures exist when the refrigerant is evaporated. At the same time, the oil which lubricates the compressor contains some refrigerant which may affect its lubricating property.
It is known in the industry that chlorodifluoromethane (known in the art as R22) and a blend of monochlorodifluoromethane and monochloropentafluoroethane (known in the art as R502) are not completely miscible in common refrigeration oils. See Downing, FLUOROCARBONS REFRIGERANT HANDBOOK, 13. A solution to this problem has been the use of alkylated benzene oils. Such oils are immiscible in R134a and are not useful therewith. This problem is most severe at low temperatures when a separated oil layer would have a very high viscosity. Problems of oil returning to the compressor would be severe.
R134a is not miscible with mineral oils: consequently, different lubricants will be required for use with R134a. However, as mentioned above, no changes to equipment should be necessary when the refrigerant substitution is made. If the lubricant separates from the refrigerant, it is expected that serious operating problems could result. For example, the compressor could be inadequately lubricated if refrigerant replaces the lubricant. Significant problems in other equipment also could result if a lubricant phase separates from the refrigerant during condensation, expansion, or evaporation. These problems are expected to be most serious in automotive air-conditioning systems because the compressors are not separately lubricated and a mixture of refrigerant and lubricant circulates throughout the entire system.
These problems have been recognized generally in the refrigeration art. Two recent publications by ASHRAE suggest that separation of lubricants and refrigerants presents problems, although no mention is made of R134a. These articles are Kruse et al, "Fundamentals of Lubrication in Refrigeration Systems and Heat Pumps," ASHRAE TRANSACTIONS 90(2B), 763 (1984) and Spauschus, "Evaluation of Lubricants for Refrigeration and Air-Conditioning Compressors," ibid, 784.
The following discussion will be more readily understood if the mutual solubility of refrigerants and various lubricating oils is considered in general, with specific reference to R134a. Small amounts of lubricants may be soluble in R134a over a wide range of temperatures, but as the concentration of the lubricant increases, the temperature range over which complete miscibility occurs, i.e., only one liquid phase is present, narrows substantially. For any composition, two consolute temperatures, i.e., a lower and a higher temperature, may exist. That is, a relatively low temperature below which two distinct liquid phases are present and above which the two phases become miscible and a higher temperature at which the single phase disappears and two phases appear again may exist. A diagram of such a system for R502 refrigerant is shown as FIG. 2 in the Kruse et al. paper mentioned above. A range of temperatures where one phase is present exists, and while it would be desirable that a refrigeration system operate within such a range, it has been found that for typical compositions, the miscible range of lubricants with R134a is not wide enough to encompass the typical refrigeration temperatures.
Some disclosures which are concerned with the choice of lubricants when R134a is used as a refrigerant exist. Polyalkylene glycols were suggested to be used in Research Disclosure 17483, October 1978 by DuPont. Specific reference was made to such oils produced by Union Carbide Corporation under the trade names ULCON LB-165 and UCON 525 (registered trademark). It is stated that these oils are miscible in all proportions with R134a at temperatures at least as low as -50.degree. C. It is believed that ULCON LB-165 and UCON 525 (registered trademark) are polyoxypropylene glycols which have a hydroxy group at one end of each molecule and a n-butyl group at the other end.
The use of synthetic oils for refrigeration systems including the class of fluorinated silicones is discussed by Sanvordenker et al. in a paper given at a ASHRAE Symposium, Jun. 29, 1972. The reference teaches that R12, chlorotrifluoromethane (known in the art as R13), R22, and R502 may be used with synthetic oils but does not teach R134a and states that the fluorinated silicones are expensive.
Commonly assigned U.S. Pat. No. 4,755,316 teaches a compression refrigeration composition. The refrigerant is tetrafluoroethane while the lubricant is at least one polyoxyalkylene glycol which is at least difunctional with respect to hydroxyl groups, has a molecular weight between 300 and 2,000, has a viscosity of about 25-150 centistokes (0.25-1.5 cm.sup.2 /sec) at 37.degree. C., has a viscosity index of at least 20, and is miscible in combination with the tetrafluoroethane in the range between -40.degree. and at least +20.degree. C. The reference does not teach the present compositions of tetrafluoroethane and fluorinated silicones. Further, the Comparatives in Table C show silicone lubricants which are immiscible with R134a and thus, the reference teaches away from the use of silicone lubricants with R134a.
U.S. Pat. No. 3,642,626 teaches polysiloxanes such as of the formula: EQU (CH.sub.3).sub.3 SiO[Si(CH.sub.3)(CH.sub.2 CH.sub.2 CF.sub.3)O].sub.n' [Si(CH.sub.3).sub.2 O].sub.n" Si(CH.sub.3).sub.3
The reference teaches that n' and n" total to the value of n which is an integer of 1 to 150 and that the preferred values for n' and n" are 20 to 75. As will be discussed later, this teaching does not suggest the present compositions of refrigerants and fluorinated silicone lubricants which are miscible with the refrigerants.
German Unexamined Patent Application 2750980 dated May 17, 1979 describes lubricants for refrigeration machines and addresses the problems of suitable lubrication at low temperatures. The reference discusses known polymeric fluorosilicone lubricants which have a high degree of polymerization. The reference specifically discloses EQU (CH.sub.3).sub.3 SiO[Si(CH.sub.3).sub.2 O].sub.10 [Si(CH.sub.3)(CH.sub.2).sub.2 C.sub.6 F.sub.13 O].sub.10 SiO(CH.sub.3).sub.3
for use with chlorotrifluoromethane (known in the art as R13) and would probably be only partially miscible or even immiscible with R134a.
U.S. Pat. No. 4,818,423 teaches that certain fluorosiloxanes are useful as lubricants with R12, R13, R502, and a blend of trifluoromethane and chlorotrifluoromethane (known in the art as R503). The fluorosiloxanes have units of alkyltrisiloxy, trialkylmonosiloxy, dialkyldisiloxy, and a substituted alkyldisiloxy which is substituted by a fluorinated moiety. The dialkyldisiloxy is the major unit present. A particular fluorosiloxane lubricant has the formula:
(alkyltrisiloxy).sub.3 (trialkylmonosiloxy).sub.5 (substituted alkyldisiloxy).sub.5 (dialkyldisiloxy).sub.50
As will be understood later, we believe that this fluorosiloxane lubricant would be immiscible with refrigerants such as R134a.
Japanese Patent Publication 96684 dated May 30, 1985 addresses the stability problems of refrigerants. The reference mentions twelve refrigerants including tetrafluoroethane. The reference also teaches six classes of lubricants including fluorinated silicone, perfluoro ether oligomer, fluorinated oxethane, chloro tri fluoro ethylene low polymer, fluorinated polyphenyl ether, and perfluoroamine.
Because it is expected that R134a will become widely used in the field of refrigeration and air-conditioning, lubricants useful with R134a are needed in the art. In our search for lubricants useful with R134a, we tested numerous lubricants including perfluoro ether oligomers and chlorotrifluoroethylenes as taught by the preceding Japanese reference and found that these lubricants were either immiscible or only partially miscible with R134a. We did not test a fluorinated oxethane as also disclosed by the Japanese because we were uncertain as to its composition. We also did not test fluorinated polyphenyl ethers and perfluoroamines as disclosed by the Japanese because we could not obtain commercial samples of these materials.
U.S. Pat. No. 4,755,316 teaches that silicones are immiscible with R134a at room temperature. We tested dimethyl/methyltrifluoropropyl siloxane with 20% methyltrifluoropropyl units as a fluorinated silicone and found that this fluorinated silicone was immiscible with R134a. We also tested dihydroxy polydimethyl siloxane and polydimethyl siloxane and found them to be immiscible with R134a at room temperature. The results of our work are in the Comparatives below.